1. Field of the Invention
The present invention relates to a semiconductor laser device suitable for use as a light source which is used for long distance, large optical communication. The present invention further relates to a method for fabricating such a semiconductor laser device.
2. Description of the Related Art
In order to realize a semiconductor laser having an improved rapid response property, which is used for long distance, large data transmission, it has been attempted to apply a quantum wire structure to an active layer of a semiconductor laser. The quantum wire structure has a high gain property relative to a quantum thin film structure. In addition, it is theoretically presumed that the semiconductor laser having the quantum wire structure operates with a small current, with high efficiency, and with narrow spectral linewidth (M. Asada et al., IEEE JQE, vol. QE-22, No. 9, pp. 1915-1921, 1986).
FIG. 6 shows a conventional semiconductor laser device 600 having a quantum wire structure (Arai et al., Proceedings of Electronics Society Conference of Institute of Electronics, Information and Communication Engineers, 1997, pp. 266-267). As shown, double-quantum well active regions 602 each having a trapezoid shape are formed above an InP substrate 601, and the width of the trapezoid is about 35 nm in the center portion thereof. The well regions 602 each have a thickness of 10 nm, and form a quantum wire structure.
A method for fabricating the conventional semiconductor laser device 600 having a quantum wire structure is described with reference to parts (a) to (c) of FIG. 7. As shown in part (a) of FIG. 7, in the first crystal growth process, an InGaAsP light confinement layer 603, a quantum well active layer 604 having two wells, and an InGaAsP protective layer 605 are serially formed on a p-type InP substrate 601. Thereafter, as shown in part (b) of FIG. 7, predetermined portions of the quantum well active layer 604 are selectively etched, thereby forming a plurality of double-quantum well active regions 602 each having a trapezoid shape in a periodic pattern. Thereafter, as shown in part (c) of FIG. 7, an undoped InP layer 606, InGaAsP light confinement layer 607, and an n-type InP cladding layer 608 are grown through a crystal growth process, thereby confining the double-quantum well active regions 602 each having a trapezoid shape.
However, in this fabrication method, it is necessary to once etch the quantum well active layer 604 so that the quantum well active layer 604 has a quantum wire structure, and faces exposed by etching are exposed to a thermal treatment during a subsequent regrowth process. Such a thermal treatment may introduce a defect by etching a portion of the quantum well active regions 602, and the defect may deteriorate an optical property of the quantum well active regions 602. Furthermore, the long-term reliability of the semiconductor laser device may decrease.
Furthermore, the size of the quantum wire structure having a trapezoid shape, which is formed by etching, varies according to even a small variation of the concentration of an etching solution. Thus, it is difficult to fabricate a quantum wire structure having a uniform size.